quantification of residual stresses in hot rolled steel

METALURGIJA 46 (2007) 1, 41-46 41

F. TREBUŇA et al.: QUANTIFICATION OF RESIDUAL STRESSES IN HOT ROLLED STEEL SHEETS BY THE ...

Received - Primljeno: 2005-04-21Accepted - Prihvaćeno: 2006-03-10

Preliminary Note - Prethodno priopćenje

ISSN 0543-5846METABK 46 (1) 41-46 (2007)

UDC - UDK 669.15–122.4–412:620.192.63=111

F. TREBUŇA, F. ŠImčáK, m. BURŠáK, J. BOCKO, P. ŠARGA, m. PáSTOR, P. TREBUŇA

F. Trebuňa, F. Šimčák, J. Bocko, P. Šarga, m. Pástor, P. Trebuňa, Faculty of mechanical Engineering Technical University of Košice, Košice, Slovakia, M. Buršák, Faculty of metallurgy Technical University of Košice, Košice, Slovakia

Quantification of residual stressesin hot rolled steel sheets by the hole drilling method

introduction

The residual stresses in sheets are the stresses that exist in object in the absence of external loading. These stresses are generated by technological process or by previous loading. In principle all technological processes - rolling, forming, thermal processing and so on - generate in produced object residual stresses [1 - 5].

The measurement of residual stresses can not be pro-vided by conventional procedures of experimental stress analysis, because the strain indicator (electrical-resistance strain gage, photoelastic coating) is absolutely insensitive to load history of examined part and it measures only strain changes after its installation. For determination of residual stresses by standard indicators, the stresses remaining within a structure have to be somehow released (with applied indicator) so that the gages register the strain change induced by stress removing. In past, this was ac-complished by destruction of examined part by cutting of material levels on the surface or by trepanation. Strain gages applied before cutting on the structure give response

The paper deals with the problems of quantification of residual stresses in hot rolled sheets produced under various regimes. On the base of stress distribution along width and thickness of the belt is pointed out on possi-bilities of belt deformation from the plane as a result of action of torsional moment as an integral internal quantity in cross-section. For the non-uniformly distributed stresses along the thickness of the sheet the new method for residual stresses determination was developed. Application of the method is described in the paper.

Key words: hot rolled steel sheets, residual stresses, drilling method

Utvrdba zaostalih naprezanja u čeličnim toplo valjanim trakama metodom bušenja. Članak se zaokuplja problemima utvrdbe zaostalih naprezanja u čeličnim toplo valjanim trakama izrađenih pri raznim režimima. Na temelju rasporeda naprezanja po debljini trake se ukazuje na mogućnost deformacije trake po ravnini, kao re-zultat djelovanja torzionog momenta kao ukupna unutarnja veličina po presjeku. Pri neravnomjernom rasporedu naprezanja po debljini trake bila je predložena nova metoda. Primjena ove metode opisana je u članku.

Ključne riječi: toplo valjane čelične trake, zaostala naprezanja, metoda bušenja

proportional to strains that are invoked by releasing of residual stresses.

The residual stresses computation work on the pre-sumption that they are elastic. many of these techniques are dedicated for laboratory applications on plane or cy-lindrical specimens and it is not easy to adopt them to real objects of arbitrary sizes and shapes. In current state of development the hole drilling strain gage method is prefer-entially used for uniform or almost uniform residual stress distribution along the depth of material. The procedures of getting data and their interpretation for such cases are well-elaborated and simple, but in special situations the treatments for verification of stress uniformity and other parameters are needed. The laboratory of authors has hardware and software for the measurement of residual stresses varying through the thickness.

The paper describes the results gained during measure-ment of residual stresses in hot rolled sheets. The sheets are produced in the form of scrolls or plates. They are further divided by customers into bands, and during this process (as a consequence of residual stresses relaxation) undesirable deformations of divided pars can occur. This phenomenon is inadmissible during processing of band on high-speed production lines and so the aim of the sheet producers is to reduce the level of residual stresses to the lowest possible size.

METALURGIJA 46 (2007) 1, 41-4642


basic data aboutchemical comPosition and materialProPerties of the inVestigated sheets

The residual stresses in hot rolled sheets produced in two different thermo-mechanical regimes with the same steel quality were determined by the hole drilling strain gage method. The first rolling technology was named as “original” the second one as “alternative”. Chemical com-position of steel sheets is given in Table 1.

In Figure 1. are shown the histograms of mechanical properties of sheets produced by original and alternative technology. Because the values of Young modulus E and

METALURGIJA 46 (2007) 1, 41-46 43


Poisson ratio m were not quoted by producer of sheets, there were according to [9], [6] used the values E = 2,05 × 105 mPa, m = 0,3.

methodology formeasurement of residual stresses

measurement and control chain consists of system for measurement of residual stresses SINT mTS 3000, strain gage measurement device, source of compressed air, and reduction valve with filter and computer with program system RESTAN.

Schema of the measurement chain is given in Figure 2. As mentioned above, the measurements of directions and levels of residual stresses were provided on materials pro-duced by two different technologies and they were carried out on specimens made from scrolls or plates. In order to have a possibility to differentiate and quantify the residual stress levels and directions, the drilling was provided as a

rule for particular location on both sides of the specimen. Bonded strain gages for both technology types are shown in Figure 3. and Figure 4.

In Figure 3. are given specimens produced by original technology of rolling taken form the scroll (Figure 3.a) and (Figure 3.b) plate sheet.

In Figure 4. are given specimens produced by alterna-tive technology of rolling taken form the scroll (Figure 4.a) and (Figure 4.b) plate sheet. For the measurement were used strain gages 1RY61 S-1.5/120 with electrical resis-tance 120 W ± 1 %, gage factor 1,94 ± 1,5 % and thermal expansion coefficient a = 10,8×10–6 °C–1.

The residual stresses computed from measured residual strains in scrolls and plates were determined on both sides of specimens and two locations along their widths. During the measurement was discovered a fact that released strains in correlation with ratio of depth of drilled hole to average diameter of gage rosette do not lie in normalized area. In accordance with ASTm E 837-01 (besides evaluation according to ASTm) further three methods for determina-tion of stresses with non-uniform stress distribution along

METALURGIJA 46 (2007) 1, 41-4644


drilled hole were used. In addition, there were provided steps allowing interpolation of measured data.

Besides of evaluation according to ASTm E 837-01 were used:- Integral method,- Kockelmann method,- method of Power Series.

Identification of locations for measurement was fol-lowing: the first symbol in labeling determines material or

production technology of sheet rolling (original or alternative). Rolled sheet produced by alter-native technology was chilled by different way. The first letter (S) determines material produced by original (old) technology of rolling. The letter (N) determines material produced by alternative (new) rolling technology. The second letter (Z) indicates scroll and the letter (T) determines plate. The third letter (H) indi-cates upper side of sheet during its movement through rolling mill while with the letter (D) is marked its underneath side.

The fourth letter determines the location of strain gage on specimen, where (S) indicates the middle part of band and (K) its boundary.

results

Residual stresses were determined in 16 locations of investigated sheets. The results of individual methods (ASTm, Integral method, Kockelmann method, method of Power Series) were put together in tables and graphs in the form of output from the system SINT mTS 3000 - system for the measurement of residual stresses. A de-tailed description of windows in system SINT mTS 3000

is given in [8]. In Figures 4. and 5. are given examples of outputs gained from the measurements. Summary of all results is con-tained in [7].

Results of measurements unambiguously showed that stresses are distributed non-uni-formly along depth of the hole and accordingly there are not fulfilled the conditions given in ASTm standards. Because the Code ASTm E 837-01 is the only one that gives standard for evaluation of residual stresses we provide readers also with results evaluated according this Code. In Figures 7., 8., 9., 10. are given magnitudes and directions of principal residual stresses on upper and underneath sides of specimens taken from scrolls

METALURGIJA 46 (2007) 1, 41-46 45


METALURGIJA 46 (2007) 1, 41-4646


produced by old (original) and new (alternative) technol-ogy. The manner of labeling of measured locations is in accordance with description given in section “methodol-

ogy for measurement of residual stresses”. In the first line in the figure are given the values according to ASTm, in the second line according to Integral method, in the third one according to Kockelmann method and the last line represents the results gained by the method of Power Series.

magnitudes and directions of principal residual stresses on upper and underneath side of plates produced by original and new technologies are summarized in Table 2. Labeling is in accordance with Figure 7. to Figure 10.

conclusion

On the base of computed principal residual stresses, their directions as well as magnitudes of equivalent stresses from measured released strains can be stated the follow-ing results:1. The magnitudes of residual stresses in sheets (also in the

case of their non-uniformity along the hole depth) pro-duced by different technologies can by unambiguously evaluated on the base of values computed according to ASTm E 837-01.

2. The values of residual stresses in specimens taken from the scrolls and determined according to ASTm E 837-01 (but also by other methods that take into account non-uniform distribution of residual stresses along the hole depth) are smaller for alternative technology of rolling than for the original one.

references

[1] ASTm E 837-01. Standard Test method for Determining Residual Stresses by the Hole-Drilling Strain-Gage method.

[2] S. Holý, a kol.: Experimentální stanovení zbytkových napětí jako podklad pro posouzení vhodného způsobu obrábění. EAN 2005, Brno, 2005.

[3] S. Holý, V. Hampl, J. Václavík, O. Weinberg: Stanovení zbytkové napjatosti - jeden z nástrojů pro výběr optimální technologie. EAN 2006, červený Kláštor, 2006, p. 169.

[4] P. macura, Z. Bembenek, B. molinek: Analýza zbytkových napětí bandážovaných válcu. EAN 2006, červený Kláštor, 2006, p. 321.

[5] P. macura, A. Fiala: Experimentální analýza zbytkových napětí na nosníku stejné pevnosti. EAN 2004, Plzeň, 2004.

[6] STN 73 1401. Navrhovanie oceľových konštrukcií, 1998. [7] F. Trebuňa, a kol.: Smery a veľkosti zvyškových napätí konštrukčných

za tepla valcovaných plechov. Záverečná správa. Košice, September 2005.

[8] F. Trebuňa, F. Šimčák: Kvantifikácia zvyškových napätí tenzomet-rickými metódami. Grafotlač, Prešov, 2005, ISBN 80-8073-227-2.

[9] F. Trebuňa, F. Šimčák: Odolnosť prvkov mechanických sústav. Emilena, Košice, 2005, ISBN 80-8073-148-9.

Acknowledgement

The project was supported by Scientific Grant Agency of Department for Education of Slovak Republic under project No. 1/2187/05 and No. 1/1073/04.

3. The scrolls produced by alternative technology of roll-ing have on their boundaries residual stresses that can result to the curling of the sheets.

4. The values of residual stresses in specimens taken from the plate produced by alternative technology are general-ly significantly smaller than they are in speci-mens from the plate produced by original technology.

5. Higher values of residu-al stresses in cross-sec-tion were determined in the middle part of plate produced by first tech-nology. The plates made by new technology have significantly smaller stresses in the middle part, but the values of one principal stress are somewhat higher on the boundary.

6. Computed values of residual stresses signalize the need of cooling regime optimization in order to decrease the values of residual stresses through whole cross-section of the sheet.

quantification of residual stresses in hot rolled steel

Documents